The present invention is directed generally to an improved braking method and system for vehicles equipped with anti-lock brake systems and traction control or anti-slip regulation systems and additional systems which provide automatic braking capabilities.
It is known to equip vehicles, especially commercial vehicles, with anti-lock brake systems (ABS) which function to prevent locking of individual wheels during vehicle braking and, thus, to shorten braking distance without losing lateral traction. In most cases, ABSs also include traction control or anti-slip regulation systems (ASR), which function to prevent spinning of the drive wheels as the vehicle starts to move by braking the spinning wheels or by throttling engine power. Thus, individual wheels of the drive axle or axles are automatically braked as needed by the ASR, without requiring the vehicle driver to actuate the brake pedal.
Additional vehicle systems which have automatic braking capabilities that function independently of driver control include adaptive cruise control systems (ACC) and rollover stability control systems (RSC).
In an ACC, the distance from the vehicle in front is automatically adjusted. If the distance from a vehicle driving in front becomes shorter than a pre-set safe distance (e.g., because the vehicle in front has slowed down), the following vehicle is automatically slowed down, without any action by the driver, either by a reduction of engine power or by automatic actuation of the brakes. DE 4200694 A1 generally discloses such an ACC.
In an RSC, automatic braking of individual wheels is initiated if the vehicle electronic control unit (ECU) senses that vehicle rollover is imminent, for example, because the vehicle is traveling too fast on a curve. If it is determined that the rollover danger is particularly great, all wheels are braked in order to decelerate the vehicle as quickly as possible. DE 19751891 A1 generally discloses such an RSC.
In vehicle systems of the types discussed above with automatic braking capabilities controlled by a vehicle ECU, the ACC and RSC permit additional braking of the vehicle by the vehicle driver via actuation of the brake pedal. As an example, this may occur in the case of an ACC if the distance to the vehicle in front is closing particularly rapidly (as in a traffic jam, for example), and the driver wishes to brake the vehicle particularly heavily as a precaution or for additional deceleration. This is particularly desirable if automatic braking by the ACC acts only on the wheels of the drive axle(s), meaning that full deceleration of the vehicle cannot be achieved (“low-cost system”). Similarly, in vehicles with RSCs, the capability of the vehicle driver to apply additional braking is desirable, especially in the case of a basic RSC in which automatic braking takes place only by the wheels of the drive axle(s).
The prior art noted above does not address additional brake actuation by the driver as described above. For example, DE 4200694 A1 (ACC) discloses that, if the driver intervenes in the driving sequence, adaptive cruise control is interrupted. This means that automatic braking is disabled and normal braking by the driver takes place. After a pre-settable time interval, adaptive cruise control is automatically reactivated. DE 19751891 A1 (RSC), also noted above, does not address the case of additional brake actuation by the driver during RSC control.
In the basic systems for ACC and RSC discussed above, only the wheels of the drive axle(s) are automatically braked in control mode. This is accomplished by the opening of one or more ASR valves, by which the driver's brake valve is bypassed, thus allowing brake fluid to flow directly from a supply tank into the brake cylinders of the drive axle(s). This brake pressure is reduced to a suitable level, which is either constant or variable, by brief opening and closing of the ABS valves disposed upstream.
If the driver additionally actuates the brake valve, the vehicle is decelerated more strongly since the front wheels are also being braked. However, if the brake pressure being injected in response to driver demand is greater than the pressure being injected automatically by the ACC or RSC, this brake pressure cannot be relayed at the desired level to the brake cylinders, since the full supply pressure is present at the closed ABS valves. Thus, further transmission of the brake pressure injected in response to driver demand is prevented, and the driver's demand for additional braking is not completely met.
It is therefore desirable to allow for additional brake actuation by the driver and to signal such actuation to the vehicle ECU to enable it to terminate automatic braking of the wheels of the drive axle(s) by closing the ASR valves and opening the ABS valves, so that the service brake pressure injected in response to driver demand can act normally on all wheels of the vehicle, including the wheels of the drive axle(s).
For this purpose, it is possible to mount a pressure sensor or a displacement sensor on the brake pedal to sense a pressure level output by the brake pedal or a travel of the pedal and to signal this information to the ECU. In addition, a further pressure sensor would be required to check whether the brake pressure exerted as a result of the driver's actuation of the brake pedal is higher than the brake pressure at the drive axle(s). Such additional sensors, including associated evaluation software, however, mean undesired higher costs.
Accordingly, it is desired to provide a method and system that provide the capability to ascertain whether braking initiated by the vehicle driver is being applied at a level greater than that resulting from automatic vehicle braking, without the need for additional hardware such as, in particular, additional sensors, and that permit the braking demands initiated by the vehicle driver to be completely met.